In industrial environments such as agriculture, mining, transportation, petrochemical drilling operations, pipeline facilities, fuel transfer facilities, or material processing plants, the atmosphere may become contaminated with airborne fuels due to leaks or spills. Airborne fuels constitute any readily vaporized, aerosolized or suspended particles of a combustible material or hydrocarbon. Exposure of an operating engine to airborne fuels is potentially hazardous, since an engine ingesting airborne fuels may run uncontrollably even if its normal fuel source is disabled, thus preventing the engine from being normally shut off. Continuous operation of the engine may trigger fires or explosions, or result in engine damage caused by improper fuel types or above-speed operation.
Compression-ignition or conventional diesel engines are particularly susceptible to this problem, since combustion is initiated by compression of the air fuel mixture in the cylinders rather than by a spark. In contrast to a spark-ignition engine, a compression ignition engine does not use a throttle valve to control intake air flow and engine speed; thus, intake air flow is typically unrestricted. Engine speed is normally controlled by adjusting the rate at which fuel is fed to the cylinders, and the engine is stopped by cutting off fuel flow. The engine can run on a wide variety of fuels including diesel, methane, natural gas, propane, gasoline, aviation fuel, aerosolized oil, H2S, grain dust, metal dust and coal dust. The engine continues to run as long as it is provided with fuel and air. If the engine is exposed to an environment where fuel is supplied to the engine externally (other than through the engine's controlled fuel system), any positive control over the engine speed may be lost. Even if the operator attempts to shut off the engine by cutting off the flow of regular fuel, the engine may run uncontrollably on the external fuel source until it is damaged or fails, possibly triggering an explosion in the rich fuel-air environment.
Positive air shutoff valves are provided to provide the ability to shut down an engine, particularly a compression-ignition engine, in routine or emergency situations. However, such valves are known to experience build-up of carbon or other deposits, which could interfere with their operation. Regular cycling of such valves is typically scheduled as part of routine maintenance. However, regular maintenance is sometimes difficult or inconvenient to perform.